GB1587610A - Process for producing chlorine dioxide - Google Patents

Description

PATENT SPECIFICATION ( 11) 1 587 610

( 21) Application No 22147/78 ( 22) Filed 24 May 1978 ( 19) N \ ( 31) Convention Application No 52/066607 ( 32) Filed 8 Jun 1977 in N ( 33) Japan(JP) 00 ( 44) Complete Specification Published 8 Apr 1981 ( 51) INT CL 3 C Oi B 11/02 _ 1 ( 52) Index at Acceptance Ci A 9 K 3 C ( 54) PROCESS FOR PRODUCING CHLORINE DIOXIDE ( 71) We, CHLORINE ENGINEERS CORP, LTD, a Japanese company of Kasumigaseki Building, No 2-5, Kasumigaseki 3-chome, Chiyoda-ku, Tokyo, Japan, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following 5

statement:-

This invention relates to a process for producing chlorine dioxide which is superior both in terms of the equipment used and the economy achieved.

Canadian Patent 782,574 discloses an electrolytic process for the production of chlorine dioxide starting with an aqueous solution of a metal chloride In view of the fact that chlorine dioxide gas in concentrated form is spontaneously explosive, specific procedures and equip 10 ment must be employed in this process in order to minimize hazards as a result thereof Thus, the equipment and process involved in conducting the process described in Canadian Patent 782,574 is quite complicated.

Chlorine dioxide is often produced by reducing a chlorate, generally sodium chlorate Known method for producing chlorine dioxide involve reducing a chlorate with a strong acid 15 such as sulfuric acid or hydrochloric acid in the presence of an alkali metal chloride.

Canadian Patent 461,586 discloses a process for producing chlorine dioxide from hydrochloric acid in which a chlorine dioxide generator comprises a plurality of reactors arranged vertically in series A liquid reaction mixture containing sodium chlorate, hydrochloric acid and sodium chloride is fed into the upper reactor and the solution cascades by gravity through 20 the remaining reactors This process for the production of chlorine dioxide is not preferred since a plurality of reactors is involved and thus the equipment design and reaction mixture flow-through are complicated The most efficient reduction method involves using hydrochloric acid as a reducing agent for the chlorate and a single reactor which also functions as a generator, an evaporator and a crystallizer, as disclosed in Canadian Patent 969,735 (corres 25 ponding to British Patent 1,347,740 and Japanese Patent Application (OPI) 15391/72) and U.S Patent 3,929,974 (corresponding to Japanese Patent Application (OPI) 59095/73).

Reduction of sodium chlorate with hydrochloric acid involves the following two reactions ( 1) and ( 2) 30 Na Cl O 3 + 2 H Cl -Cl O 2 +LC 12 + Na CI + H 20 ( 1) Na Cl O 3 + 6 H ClNa Cl + 3 C 12 + 3 H 20 ( 2) To obtain chlorine dioxide with good efficiency, it is desirable to select reaction conditions 35 which will promote reaction ( 1) while inhibiting reaction ( 2), for example by using a suitable catalyst or by selecting the proportions of the starting materials fed into the reactor such that reaction ( 1) will proceed mainly.

Sometimes, as described above to utilize sodium chloride in the spent reaction solution effectively, the recovered sodium chloride is fed as a starting material to an electrolytic cell 40 for production of sodium chlorate.

For example, Canadian Patent 825,084 discloses a process for the production of chlorine dioxide and chlorine and the production of an alkali metal salt in a single vessel by utilizing as starting materials an alkali metal chlorate, an alkali metal chloride and a strong acid in suitable proportions to generate chlorine dioxide and chlorine However, after removal of the 45 1,587,610 chlorine dioxide and chlorine generated in the reaction together with water vapor, the reaction mixture in the reaction vessel must be maintained at a sufficiently high temperature that water is removed from the reaction medium in order to crystallize out the alkali metal salt of the strong acid employed which can then be withdrawn and recovered from the reaction vessel as the alkali metal salt Since an alkali metal chloride is often utilized as the material for 5 production by electrolysis of the starting alkali metal chlorate, in the process of Canadian Patent 825,084 only if hydrochloric acid is employed as the strong acid can recycling of the alkali metal chloride to an electrolytic cell for the production of alkali metal chlorate be considered and further without removal from the alkali metal salt recovered from the reaction system, after evaporation of water therefrom, of residual chlorine dioxide and 10 chlorine gas can the alkali metal chloride be recycled to an electrolytic cell for the production of chlorine dioxide without explosion hazards arising.

Further, Canadian Patent 826,577 discloses a process similar to that of Canadian Patent 825,084, with the exception that sulfuric acid is employed as the strong acid and sodium sulfate is formed as a reaction product Thus, the alkali metal salt formed, sodium sulfate, 15 cannot be recycled to an electrolytic cell for the production of sodium chlorate for utilization as a starting material in the production of chlorine dioxide and chlorine.

The method disclosed in Canadian Patent 969,735 comprises reacting hydrochloric acid with sodium chlorate in a reactor to form chlorine dioxide and chlorine, reducing the pressure of the reactor at that time, evaporating water at a temperature sufficient to crystallize sodium 20 chloride, withdrawing a gaseous mixture of steam, chlorine and chlorine dioxide from the reactor, simultaneously recovering the solid sodium chloride precipitated, electrolyzing the recovered sodium chloride as an aqueous solution, and feeding the resulting sodium chlorate to the reactor mentioned above.

When the above method is operated under conditions which will precipitate sodium 25 chloride, the slurry-like residual reaction mixture containing precipitated sodium chloride is withdrawn from the bottom of the reactor The solution resulting from removal of the sodium chloride is recycled to the reaction, and in the meantime, the solid sodium chloride is washed with water and re-dissolved, and fed into an electrolytic cell for the production of sodium chlorate The solution of sodium chloride is electrolyzed for form sodium chlorate, and the 30 sodium chlorate is fed into the reactor.

If the slurry-like residual reaction mixture containing the precipitated sodium chloride is fed continuously to the electrolytic cell for the production of sodium chlorate without the separating treatment, the unreacted hydrochloric acid remains in the residual reaction mixture and the p H of the residual reaction mixture becomes about 3 to 4 or lower This tends 35 to cause the generation of chlorine gas during the electrolysis in the electrolytic cell for production of sodium chlorate Furthermore, since the chlorine dioxide and the chlorine present in the residual reaction mixture flow into the electrolytic cell, the chlorine can react with the hydrogen generated in the electrolytic cell giving rise to explosion hazards.

For this reason, this process described above requires a step for separating the residual 40 reaction mixture and a step for washing and re-dissolving the sodium chloride, and therefore, the process steps become complicated Moreover, according to this process, a reservoir for the residual reaction mixture is provided beneath the reactor to prevent a clogging of the withdrawal pipe because the withdrawal of the residual reaction mixture from the reactor is conducted intermittently, the reactor is at a reduced pressure, and the residual reaction 45 mixture withdrawn contains a slurry of sodium chloride Hence, the reactor must be installed at a high level above the ground, and the apparatus as a whole becomes large in size.

U.S Patent 3 929,974 discloses a process for producing chlorine dioxide by continuously feeding an aqueous solution of an alkali metal chlorate and hydrochloric acid into a reaction zone where chlorine dioxide and chlorine are continuously formed by the reaction between 50 the alkali metal chlorate and the hydrochloric acid During the reaction, the reaction medium is maintained at its boiling point to evaporate water from the reaction medium to form a gas phase in the reaction zone consisting of a mixture of chlorine dioxide, chlorine and water vapor which is then removed from the reaction zone Two embodiments of the process are disclosed, one in which sodium chloride produced in the reaction is not precipitated but 55 rather is removed as a liquid effluent continuously to maintain the liquid level in the reaction zone constant and another in which solid sodium chloride as a precipitate is removed from the reaction zone In the first embodiment in which a liquid medium containing the sodium chloride is removed, such also contains dissolved therein chlorine dioxide and chlorine which is not stripped in the reaction vessel, and due to the presence of these materials a series of side 60 reactions occur, some which are particularly dangerous when large quantities of hydrogen are mixed with the chlorine dioxide and chlorine and which reduce the efficiency of the electrolytic cell used to convert sodium chloride into sodium chlorate In the second embodiment, in which solid sodium chloride is removed from the reaction zone, such is continuously dissolved in water and then fed to an electrolytic cell for the production of chlorate where it is converted 65 1,587,610 into sodium chlorate and hydrogen Unfortunately, just as was the situation with Canadian Patent 969,735, this mixture containing solid sodium chloride removed from the reaction zone will also contain chlorine dioxide and chlorine which, when passed into an electrolytic cell, the chlorine can react with hydrogen generated in the electrolytic cell to give rise to explosion hazards 5 An object of this invention is to obviate or mitigate the above described problems, and to produce chlorine dioxide advantageously both from the standpoint of the equipment involved and the economy obtained without the need for a complicated and large-scaled process.

The present invention provides a process for producing chlorine dioxide which comprises 10 the steps of:

( 1) feeding hydrochloric acid and a stoichiometric excess of sodium chlorate produced in an electrolytic cell for producing sodium chlorate into a reaction zone for producing chlorine dioxide; ( 2) reacting the hydrochloric acid with the sodium chlorate in the reaction zone to form a 15 gaseous reaction product containing chlorine dioxide and chlorine and so that sodium chloride produced is precipitated to form a slurry-like residual reaction mixture containing the precipitated sodium chloride; ( 3) continuously removing the chlorine dioxide as the gaseous reaction product; ( 4) simultaneously withdrawing the slurry-like residual reaction mixture containing the 20 precipitated sodium chloride continuously from the reaction zone; ( 5) feeding water to the withdrawn residual reaction mixture to dissolve the sodium chloride and form a solution containing sodium chloride; ( 6) blowing air or an inert gas (as defined herein) through the solution containing sodium chloride to remove any residual chlorine dioxide and chlorine from the solution; and 25 ( 7) then recycling the solution to an electrolytic cell for the production of sodium chlorate.

The term "inert gas" as used herein means a gas that does not undergo chemical reaction with chlorine or chlorine dioxide under any of the process conditions.

The process may additionally include the step of feeding the residual chlorine dioxide and 30 chlorine removed by blowing air or an inert gas through the solution containing the sodium chloride into the reaction zone for production of chlorine dioxide.

The accompanying drawing is a flow diagram illustrating one embodiment of a process of the present invention.

Since, in the present invention, hydrochloric acid is reacted with a stoichiometric excess of 35 sodium chlorate, the hydrochloric acid is almost completely consumed in the reaction, and the amount of hydrochloric acid contained in the solution containing sodium chloride formed by adding water to the withdrawn residual reaction mixture is small, e g 5 g/l or less, more generally 0 5 g/l or less As a result, the p H of the solution containing sodium chloride formed by adding water to the withdrawn residual reaction mixture is maintained at about 3 40 to 4 or higher, and chlorine gas is not generated when the solution containing sodium chloride formed by adding water to the withdrawn residual reaction mixture is electrolyzed in the electrolytic cell for the production of sodium chlorate Furthermore, air or an inert gas is blown through the solution containing sodium chloride formed by adding water to the withdrawn residual reaction mixture from the reactor for production of chlorine dioxide to 45 remove chlorine dioxide gas and chlorine gas dissolved in this solution, and the remainder is fed into the electrolytic cell for the production of sodium chlorate For this reason, there is no risk of an explosion occurring due to a reaction between the chlorine gas generated in the electrolytic cell or chlorine gas which flows into the electrolytic cell and hydrogen gas Thus, the step of separating the solid sodium chloride from the solution containing sodium chloride 50 formed by adding water to the withdrawn residual reaction mixture necessary in the prior art is not required in the process of this invention.

Referring now to the drawing reference numeral 1 represents a reactor for the production of chlorine dioxide Hydrochloric acid and sodium chlorate as starting materials and as aqueous solutions are fed separately into the reactor, with the sodium chlorate being fed in a 55 stoichiometric excess amount, e g in a molar ratio of the hydrochloric acid to the sodium chlorate generally of 0 3: 1 to less than 2: 1 preferably 0 5: 1 to 1: 1, and are reacted in the reactor.

A suitable concentration of hydrochloric acid which can be used is 200 to 400 g/e, preferably 350 g/e and a suitable concentration of sodium chlorate which can be used is 300 60 to 600 g/e preferably 450 to 550 g/e The hydrochloric acid aqueous solution is generally fed at room temperature ( 20 30 WC) and the sodium chlorate aqueous solution is generally fed at temperature of from 60 to 90 WC The reactor is maintained at reduced pressure, and water is evaporated at a temperature sufficient to precipitate sodium chloride as a solid salt, e g, at a temperature of 65-800 C in the reactor The resulting chlorine dioxide and chlorine 65 1,587,610 are obtained using steam as a carrier Thus, this single reactor has the function of a generator, an evaporator and a crystallizer, and is of the type disclosed, for example, in Canadian Patent 825,084.

In an electrolytic cell 2 for the production of sodium chlorate, an aqueous solution of sodium chloride is electrolyzed, and the resulting aqueous solution of sodium chlorate is fed 5 into a reservoir 3 Descriptions of electrolytic cells which can be used are set forth in U S.

Patent 3,350 286 A suitable electrolytic cell voltage is 3 to 5 V, a suitable current density is 5 to 20 A/dmi and a suitable temperature for the electrolysis is about 450 C or higher up to about the boiling point of the electrolyte, preferably 60 to 80 'C A suitable concentration of the sodium chloride aqueous solution in the electrolysis is 50 g/e or more, preferably 100 to 10 300 g/e, and in general the concentration of the sodium chlorate at the outlet of the electrolytic cell after continuous operation (i e, circulation of sodium chlorate) is 300 to 600 g/l.

The aqueous solution of sodium chlorate is then fed from reservoir 3 to the reactor 1 In the meantime, hydrochloric acid is fed from a reservoir 4 into the reactor 1 15 It is important for the amount of sodium chlorate fed to be in excess of the stoichiometric amount for reaction between the sodium chlorate and hydrochloric acid to permit substantially complete consumption of the hydrochloric acid in the reaction.

Desirably, sodium chlorate and hydrochloric acid are reacted while maintaining the reactor 1 at a reduced pressure of 100 to 700 mm Hg, preferably 100 to 300 mm Hg, and 65 to 800 C 20 which is the boiling point of the solution at this pressure These conditions are determined because it is preferred to maintain the reaction system at reduced pressure so as to promote the generation of a vapour and cause the chemical equilibrium in the reaction for generating chlorine dioxide to shift toward the generation of chlorine dioxide, and it is necessary to perform the reaction at a temperature below the point (about 850 C) at which chlorine dioxide 25 substantially decomposes A catalyst is not necessary for this reaction but, if one is used silver ion, mangasese ion, chromium ion and the like can be suitably used in the form of fine powders thereof The conditions of a high temperature (e g, 65 to 80 WC) and a reduced pressure (e g, 100 to 700 mm Hg, preferably 100 to 300 mm Hg) described above and used in this step also cause the precipitation of sodium chloride 30 The resulting chorine dioxide and chlorine are withdrawn as a gaseous product 5 together with the steam Although it often is not necessary to separate the chlorine dioxide from the mixture of chlorine dioxide and chlorine gas obtained in this invention, if desired, the chlorine dioxide and chlorine gas may be separated A suitable separation procedure comprises passing the gaseous mixture through cold water (e g, at 5 OC or less) whereby chlorine dioxide 35 dissolves in the cold water but the chlorine gas does not dissolve in the cold water.

The residual reaction mixture containing precipitated solid sodium chloride, dissolved sodium chloride unreacted sodium chlorate unreacted hydrochloric acid and dissolved chlorine dioxide and chlorine is withdrawn from the reactor 1 through the inlet pipe of a 40 thermosiphon-type reboiler beneath the reactor 1 while the liquid level within the reactor 1 is maintained constant A part of the residual reaction mixture is heated by reboiler 6, and then recycled to the reactor with the remainder of the residual reaction mixture being circulated to the electrolytic cell 2 after having been subjected to water treatment and gas treatment Since hydrochloric acid is consumed almost completely in the reaction and is present only in a very 45 small amount as described hereinbefore in the residual reaction mixture, the p H of the residual reaction mixture is maintained at about 3 to 4 or higher Water 10 heated, e g, to a temperature of 50 to 80 WC preferably 70 to 750 C by a heat exchanger 9 is continuously fed into the residual reaction mixture withdrawn from the reactor 1 so as to dissolve the solid sodium chloride Then, this mixture is fed into a tank 7 maintained at the same pressure as the 50 reactor through an equalizing line (a ventilated pipe for causing the pressure to be uniform) 11, and stirred to dissolve the solid sodium chloride and form a solution containing sodium chloride Simultaneously, the dissolved chlorine dioxide gas and chlorine gas are removed by air 12 from which oil has been removed or an inert gas (e g, nitrogen gas) and which is blown through the bottom of the tank 7 55 The chlorine dioxide and chlorine removed are recycled to the reactor 1 through the equalizing line 11 and are effectively used and pollution by the chlorine dioxide and the chlorine is prevented.

Tank 7 is preferably maintained at a temperature above a certain point in order to facilitate the removal of the dissolved gases in the solution containing sodium chloride and the 60 dissolving of the precipitated salt For example, good results can be obtained by operating the reactor 1 at 100 to 700 mm Hg preferably 100 to 300 mm Hg, and 65 to 80 WC, adjusting the temperature of the hot water fed to the withdrawn residual reaction mixture at 55 to 70 WC, and operating the tank 7 at 50 to 650 C.

The sodium chloride-containing solution resulting from the abovedescribed procedure 65 1,587,610 5 which will generally contain a maximum of 100 ppm of a mixture of chlorine gas and chlorine dioxide is fed into a reservoir 8 from where it is recycled to an electrolytic cell 2 for production of sodium chlorate.

Since in the present invention hydrochloric acid and sodium chlorate are reacted with the sodium chlorate being present in an excessive proportion to permit substantially complete 5 consumption of the hydrochloric acid in the reaction, the p H of the solution containing sodium chloride to be recycled to the electrolytic cell for production of sodium chlorate can be maintained at 3 to 4 or higher Hence, the likelihood of chlorine gas generation by electrolysis in the electrolytic cell for producing sodium chlorate is substantially minimized.

Furthermore, since in the present invention, chlorine dioxide and chlorine in the solution 10 containing sodium chloride are removed by blowing air or an inert gas through the solution, gases do not flow into the electrolytic cell for production of sodium chlorate even if solid salt in the solution is not separated and washed Consequently, there is no risk of explosion by reaction of hydrogen and chlorine in the electrolytic cell.

The present invention thus does not require the steps of separating and washing of the solid 15 sodium chloride in the residual reaction mixture as in the prior art, and therefore, equipment necessary for these steps can be omitted It is also unnecessary to provide a device for withdrawing the residual reaction mixture and separating the solid sodium chloride intermittently at a position below the reactor Hence, the process as a whole can be simplified and the scale is small, and the withdrawal of the residual reaction mixture and the feeding of the 20 solution containing sodium chloride to the electrolytic cell for production of sodium chlorate can be performed continuously.

In addition, sodium chloride, chlorine dioxide and chlorine present in the solution containing sodium chloride in the production of chlorine dioxide can be effectively utilized.

Chlorine dioxide produced by the process of this invention can be used for bleaching of 25 pulp and in other applications.

The following Example is given to illustrate the present invention in greater detail Unless otherwise indicated, all parts percents, ratios and the like are by weight.

EXAMPLE

Chlorine dioxide was produced under the following conditions using a titanium-made 30 cylinder (inside diameter 200 mm x height 1000 mm) as a chlorine dioxide generating reactor 1, an electrolytic cell (bipolar electrode type, 65 Amp) as a sodium chlorate-producing electrolytic cell 2, a water ejector made of titanium, and a double tube type heat exchanger having a titanium inside pipe as a heat exchanger 9.

( 1) Electrolysis Conditions inthe Electrolytic Cell 2 for Production of Sodium Chlorate: 35 Electrolytic Cell: bipolar electrode-type, 65 Amp.

Current Density: 30 A/dinm 2 Electrolysis Temperature: 55 C Current Efficiency: 90 % ( 2) Conditions for Production of Chlorine Dioxide in Reactor 1: 40 Hydrochloric Acid Aqueous Solution Fed to Reactor 1 Flow Rate: 1 9 liters/hr.

Temperature: 20 C HC 1 Concentration: 345 9 g/liter Aqueous Solution of Sodium Chlorate Fed to Reactor 1 45 Flow Rate: 6 0 liters/hr.

Temperature: 60 C Na CIO 3 Concentration: 597 g/liter Na CI Concentration: 117 g/liter Reaction Pressure in Reactor 1: 200 mm Hg (abs) 50 Reaction Temperature in Reactor 1: 74 C ( 3) Conditions for Treating the Residual Reaction Mixture:Residual Reaction Mixture Flow Rate: 4 7 liters/hr.

Temperature: 75 C 55 Na CIO 3 Concentration: 572 g/liter Na CI Concentration: 148 g/liter (a saturated solution of 10 %by weight slurry) Hot Water Fed to the Residual Reaction Mixture Flow Rate: 1 6 liters/hr.

Temperature: 60 C 60 Air Blown into the Sodium Chloride Containing Mixture Flow Rate: 200 liters/hr.

Treated Sodium Chloride-Containing Mixture Fed into the Electrolytic Cell for Production of Sodium Chlorate:Flow Rate: 6 5 liters/hr 65 1,587,610 1.587,610 Temperature: 60 WC Na CIO 3 Concentration: 416 g/liter Na CI Concentration: 181 g/liter ( 4) Gases Produced from Reactor 1:Composition: C 102500 g/hr 5 C 12380 g/hr.

Steam 2842 g/hr.

Temperature: 750 C Pressure: 200 mm Hg (abs)

Claims (1)

1. WHATWECLAIMIS: 10

   1 A process for producing chlorine dioxide which comprises the steps of:

   ( 1) feeding hydrochloric acid and a stoichiometric excess of sodium chlorate produced in an electrolytic cell for producing sodium chlorate into a reaction zone for producing chlorine dioxide; ( 2) reacting the hydrochloric acid with the sodium chlorate in the reaction zone to form a 15 gaseous reaction product containing chlorine dioxide and chlorine and so that sodium chloride produced is precipitated to form a slurry-like residual reaction mixture containing the precipitated sodium chloride; ( 3) continuously removing the chlorine dioxide as the gaseous reaction product; 4 simultaneously withdrawing the slurry-like residual reaction mixture containing the 20 precipitated sodium chloride continuously from the reaction zone; ( 5) feeding water to the withdrawn residual reaction mixture to dissolve the sodium chloride and form a solution containing sodium chloride; ( 6) blowing air or an inert gas (as defined herein) through the solution containing sodium chloride to remove any residual chlorine dioxide and chlorine from the solution; and 25 ( 7) then recycling the solution to an electrolytic cell for the production of sodium chlorate.

   2 The process of claim 1 further including the step of feeding the chlorine dioxide and the chlorine removed from the dissolved reaction mixture by blowing the air or the inert gas therethrough into the reaction zone for production of chlorine dioxide 30 3 The process of claim 1 or 2 wherein the molar ratio of the hydrochloric acid to the sodium chlorate fed is 0 3: 1 to less than 2: 1.

   4 The process of claim 1, 2 or 3 wherein the hydrochloric acid and the sodium chlorate are fed as aqueous solutions thereof at a concentration of 200 to 400 g/l and 300 to 600 g/l, respectively 35 The process of claim 1, 2,3 or 4, wherein the reacting of the hydrochloric acid and the sodium chlorate in the reaction zone is at a temperature of 65 to 80 WC under a reduced pressure of 100 to 700 mm Hg.

   6 The process of any preceding claim, wherein the continuous removal of the chlorine dioxide as the gaseous product is effected using steam 40 7 The process of any preceding claim, wherein the water fed to the withdrawn residual reaction mixture to dissolve the sodium chloride is at a temperature of 55 to 700 C.

   8 A process for producing chlorine dioxide substantially as hereinbefore described with reference to the accompanying drawing.

   9 Chlorine dioxide when produced by the process as claimed in any preceding claim 45 Agents for the Applicants:MARKS & CLERK, Alpha Tower, ATV Centre, 50 Birmingham B I TT.

   Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1981.

   Published by The Patent Office, 25 Southampton Buildings London, WC 2 A IAY, from which copies may be obtained.